The present invention relates to a device for measuring capacitance, and more particularly to a device to detect the edge of materials, and even more particularly, to a device for the contactless detection of a moving web of material.
U.S. Pat. Nos. 5,394,095 and 5,767,686 describe and illustrate devices that detect the edge of conductive materials and control the position of the conductive materials relative to the detector of the type to which the present invention is directed. These patents are incorporated by reference herein as background information to explain certain prior edge detection and web position control arrangements of which the present invention is a specific improvement. Consequently, the details of the conductive material and the positioning of the conductive material to which the present invention is directed need not be explained in detail.
The present invention pertains to a device to detect the capacitance between two or more electrodes as a material is positioned between the electrodes, and more particularly to an edge detector for contactless detection of the position of a moving web of conductive material and will be described with particular reference thereto; however, the invention has broader applications and can be used in any type of application which requires the accurate detection of capacitance between two or more electrodes.
Edge detectors are commonly used in conjunction with apparatus that process moving webs of material. The edge detector is used to properly align the moving web of material in the apparatus. Several types of mechanical and electronic devices have been used to detect and/or control the position of the web of material. Some of these devices are disclosed in U.S. Pat. Nos. 5,394,095 and 5,767,686. In these prior art devices, the lateral position of the web of conductive material is detected by positioning two or more detector electrodes in the area of each lateral edge of the conductive material. The capacitance between the electrodes at each lateral edge of the conductive material is measured and then used to control the lateral position of the conductive material. The lateral movement of the conductive material between the electrodes causes changes in the coupling fields between the electrodes, thereby increasing or decreasing the capacitance between the electrodes. The measured capacitance is used to determine the position of the edge of the conductive material relative to the electrodes, and such determined position is used to control the lateral position of the conductive material. The electrodes are typically shielded to isolate the electrodes from external electrical interference so as to improve the accuracy of the measured capacitance.
Although these devices satisfactorily measure the capacitance between the electrodes to determine the position of the edge of the conductive material relative to the electrodes, these devices are expensive and complex to manufacture, are very sensitive to external electrical interferences, and are unable to account for parasitic charges in the measuring circuit, which parasitic charges can adversely affect the accuracy of the edge measurement and cause undesired drift in the position control of the conductive material.
In view of the present art of capacitance measuring devices for the detection and the position control of the edge of a conductive material, there is a need for a device that accurately measures the capacitance between two or more electrodes, is less sensitive to external electrical interference, and reduces or overcomes the drift in the device.
In accordance with the present invention, there is provided a capacitance measuring circuit which is designed to measure the capacitance between a sensor electrode and a reference electrode. The capacitance measuring circuit can be used in many different types of applications where it is desired to accurately measure capacitance between two or more electrodes. The capacitance measuring circuit has particular applicability for use in detecting an edge of a conductive material, and will be particularly described with reference thereto; however, the capacitance measuring circuit can be used to measure the edge of a non-conductive material. The sensor electrode and the reference electrode of the capacitance measuring circuit are spaced apart from one another a sufficient distance so that an edge of a material can pass there between. Typically, the sensor electrode and the reference electrode are symmetrically oriented about a central axis that runs between the electrodes; however, it is not necessary that the electrode be symmetrically oriented with respect to one another. The capacitance measuring circuit includes or is designed to be connected to a first polarity voltage source and a second polarity voltage source. The first and second polarity voltage sources are used to electrically charge the sensor electrode and/or reference electrode. The capacitance measuring circuit also includes a capacitor which is used to fully or partially store a charge that is fully or partially representative of the capacitance between the sensor electrode and the reference electrode. The capacitance measuring circuit further includes a switch controller to electrically connect and/or disconnect the reference electrode, the sensor electrode, the first and second polarity voltage sources and/or the capacitor to or from one another. This sequence of operations by the switch controller to measure the capacitance between the reference electrode and sensor electrode is termed the capacitance measuring sequence. The term xe2x80x9celectrically connectedxe2x80x9d as used in this application refers to an electrical connection that allows current and/or voltage to pass between two or more components in an electrical circuit. The term is not to be interpreted as requiring two or more components to be directly connected, thus two or more xe2x80x9celectrically connectedxe2x80x9d components can be directly electrically connected or indirectly electrically connected. Two or more components that are indirectly electrically connected include one or more electrical elements connected between the two or more components. These electrical elements include, but are not limited to, resistors, inductors, capacitors, amplifiers, microprocessors, diodes and the like.
In accordance with one aspect of the present invention, the first polarity voltage source is a positive voltage source and the second polarity voltage source is a negative voltage source. The first polarity voltage source and the second polarity voltage source can be provided by one or more voltage sources. In one embodiment, the voltage source is a D.C. voltage source. In another embodiment, the voltage source is a rectified A.C. source. In yet another embodiment, the magnitude of the first polarity voltage source and the second polarity voltage source are generally equal. When the magnitude of the voltage sources are equal, the resulting charge on the capacitor during each measurement sequence actually or nearly represents the actual capacitance between the sensor electrode and the reference electrode. However, different magnitude voltage sources can be used. When different magnitude voltage sources are used, the charge on the capacitor during each measurement cycle will represent a relative capacitance. This relative capacitance can be used to control the position of the material or be converted to an actual capacitance prior to being used to control the positioning of the material.
In accordance with another aspect of the present invention, the conductive material is a web of material having a generally constant width and two lateral edges. At least one of the lateral edges of the material is oriented to be at least partially positioned between the sensor electrode and reference electrode. The material is made of a material to cause an interference in the electrical field between the sensor electrode and the reference electrode when a voltage is applied to the sensor and/or reference electrode. This interference in the electrical field alters the capacitance between the sensor electrode and reference electrode. As a result, the measured capacitance between the sensor electrode and reference electrode is representative of the position of the edge of the material relative to the sensor and/or reference electrode. Therefore, the position of the material can be controlled by the detected capacitance. The type of material used is not critical. The material may be a conductive or non-conductive material. The material needs only to alter the capacitance between the reference and the sensor electrode.
In accordance with still another aspect of the present invention, the switch controller causes the capacitor in the capacitance measuring circuit to be charged and at least partially discharged during a capacitance measuring sequence so that the remaining charge on the capacitor represents the capacitance between the sensor electrode and the reference electrode. The switch controller causes a special sequence of electrical connections and disconnections to occur in the circuit thereby causing the charge on the capacitor to represent the capacitance between the reference and sensor electrode. In one embodiment, the switch controller causes the capacitor to be charged by a first polarity voltage on the sensor electrode which voltage constitutes a voltage between the universe and the sensor electrode, and to be discharged by a second polarity voltage that is similar on the sensor and reference electrode. In accordance with one aspect of this embodiment, the switch controller resets the capacitance measuring circuit after one or more capacitance measuring sequences. Typically, the capacitance measuring circuit is reset by grounding and/or draining the charge from the capacitor so the capacitor""s charge is substantially zero. In another aspect of this embodiment, the switch controller, at the beginning of the capacitance measuring sequence, electrically isolates the capacitor from the sensor electrode and reference electrode. The switch controller, simultaneously or subsequently isolates the sensor electrode from the reference electrode and grounds the reference electrode. As a result, the capacitor, the reference electrode and the sensor electrode are isolated from one another. Once these components have been isolated, the switch controller electrically connects the sensor electrode to the first polarity voltage source. The first polarity source charges the sensor electrode to substantially the same voltage or potential as the first polarity voltage source. During this charging sequence, the reference electrode is grounded and the capacitor is maintained in electrical isolation from the reference and sensor electrode. After the sensor electrode is charged, the switch controller electrically disconnects the sensor electrode from the first polarity voltage source. At this point of the capacitance measuring sequence, the reference electrode and the material are grounded and the sensor electrode is charged to substantially the same voltage or potential as the first polarity voltage source. The capacitance measuring sequence is continued by the switch controller electrically connecting the sensor electrode to the capacitor so that the charge on the sensor electrode is transferred to the capacitor. Once the charge from the sensor electrode is transferred to the capacitor, the switch controller electrically disconnects the sensor electrode from the capacitor. The switch controller then causes the reference electrode to have a potential or voltage that is at least substantially equal to the potential or voltage on the sensor electrode and disconnects the reference electrode from the grounded state. The connection formed at the reference electrode causes the potential on the reference electrode to follow the potential on the sensor electrode. The switch controller then connects the opposite polarity voltage source to the sensor electrode. The sensor electrode is then charged to the voltage or potential of the opposite polarity voltage source. The potential on the reference electrode follows the potential on the sensor electrode. Once the sensor electrode has been charged by the opposite polarity voltage source, the switch controller disconnects the opposite polarity voltage source from the sensor electrode. The capacitance measuring sequence is continued by the switch controller electrically connecting the sensor electrode to the capacitor. The charge on the sensor electrode is then transferred to the capacitor. Since the polarity of the voltage on the sensor electrode is opposite the polarity of the voltage previously supplied by the sensor electrode, the capacitor is fully or partially discharged as the charge on the sensor electrode is transferred to the capacitor. The remaining charge on the capacitor after a single capacitance measuring sequence represents the capacitance between the reference electrode and the sensor electrode. As can be appreciated, if the material is completely obscuring the reference electrode and the sensor electrode, the resulting charge on the capacitor after a single capacitance measuring sequence is substantially zero. Furthermore, the resulting charge on the capacitor will be smallest when the material is fully positioned between the reference electrode and sensor electrode. Consequently, the further the edge of the material moves between the reference electrode and the sensor electrode, the smaller the resulting charge on the capacitor after each capacitance measuring sequence. This charging and discharging of the capacitor has been found to accurately measure the capacitance between the reference electrode and the sensor electrode when a material is at least partially positioned there between. In another embodiment, each capacitance measuring sequence is less than about one second. Typically, the faster the capacitance measuring sequence, the better the position control of the material. In one aspect of this embodiment, each capacitance measuring sequence is much less than one second, and preferably about 10 xcexcs (10xe2x88x925 second).
In accordance with still yet another aspect of the present invention, the charge on the capacitor is integrated over a series of capacitance measuring sequences so as to obtain an average charge on the capacitor over a series of capacitance measuring sequences. In one embodiment, the capacitor is included in an integrator circuit wherein the charge on the capacitor is accumulated over a series of capacitance measuring sequences prior to being drained or discharged. In one aspect of this embodiment, a charge detector detects the charge on the capacitor and monitors the number of capacitance measuring sequences which have transpired. The charge detector measures the accumulated charge on the capacitor and divides the detected charge by the number of capacitance measuring sequences to obtain the average capacitance between the reference electrode and the sensor electrode. After the charge on the capacitor has been integrated over a number of capacitance measuring sequences, the capacitor is drained or discharged so as to reset the charge of the capacitor to zero to begin the recharging of the capacitor by one or more capacitance measurement sequences. In another aspect of this embodiment, the charge detected by the charge detector is converted from an analog signal to a digital signal. The conversion of the signal from analog to digital facilitates in the subsequent mathematical manipulation of the signal. For instance, the digital signal can be used in a microprocessor and/or other digital circuit to control the lateral position of the conductive material, used for calibration purposes, used to zero-out the values in the circuit, etc. As can be appreciated, the conversion of the signal from an analog to a digital signal is not required for the operation of the circuit. In still another aspect of this embodiment, the capacitor has a rated capacitance which is greater than the measured capacitance between the sensor electrode and reference electrode. Typically, the capacitance rating of the capacitor is at least about five times greater than the capacitance between the reference electrode and the sensor electrode, preferably at least about ten times greater than the capacitance between the reference electrode and the sensor electrode, and more preferably about 50-300 times the capacitance between the reference electrode and the sensor electrode. The large capacitance rating on the capacitor relative to the measured capacitance between the reference electrode and the sensor electrode ensures that an accurate capacitance reading is obtained. Furthermore, the large capacitor rating allows the charge on the capacitor to be integrated over a number of capacitance measuring sequences without fully charging the capacitor.
In a further aspect of the present invention, the sensor electrode and/or the reference electrode are shielded so as to reduce external electrical interferences on the electrodes when measuring the capacitance between the reference electrode and the sensor electrode. In one embodiment, the sensor electrode is shielded. Shielding of the reference electrode is not required to obtain accurate capacitance readings. In one aspect of this embodiment, the shielding arrangement for the sensor electrode is similar to that disclosed in Kespohl U.S. Pat. No. 5,767,68. In another embodiment, the reference electrode and the sensor electrode are shielded. When large voltages are applied to the sensor electrode and the reference electrode and/or when very accurate capacitance readings are required, the reference electrode and the sensor electrode are typically shielded. In one aspect of this embodiment, the shielding arrangement for the reference electrode and sensor electrode is similar to that disclosed in Kespohl U.S. Pat. No. 5,767,686.
In accordance with yet a further aspect of the present invention, the capacitance measuring circuit includes a system parasitic controller to compensate for the parasitic charge which develops in the capacitance measuring circuit. During the operation of the capacitance measuring circuit, a residual or parasitic charge can develop from one or more components of the capacitance measuring circuit. This residual or parasitic charge can exist even after one or more of the components of the capacitance measuring circuit have been reset and/or grounded. However, this residual or parasitic charge is most prevalent when the charge on the capacitor is being integrated over multiple capacitance measuring sequences. This residual or parasitic charge can result in variances in the measured charged during one or more capacitance measuring sequences. As a result, the parasitic charge can result in drift in the position control mechanism for the material and/or cause an unstable position control system for the material. The parasitic charge also can cause inaccurate position information of the material relative to the reference electrode and sensor electrode. The system parasitic controller is designed to periodically measure the parasitic charge in the capacitance measuring circuit. In one embodiment, the system parasitic controller measures the parasitic charge after each capacitance measuring sequence. In another embodiment, the system parasitic controller measures the parasitic charge after a plurality of capacitance measuring sequences. In one aspect of this embodiment, the system parasitic controller measures the parasitic charge at least once every minute, and preferably at least once every 30 seconds, and more preferably at least once every 10 seconds. In yet another embodiment, the system parasitic controller integrates the measured parasitic charge over a plurality of system parasitic controller measuring sequences. In still another embodiment, the system parasitic controller adjusts the measured charges on the capacitor with the measured parasitic charge so as to reduce or eliminate the effects of the parasitic charge from the measured charges on the capacitor. In one aspect of this embodiment, the measured parasitic charge is subtracted from the measured charge on the capacitor. In still yet another embodiment, the measured parasitic charge is converted from an analog signal to a digital signal so that the measured parasitic charge can be accounted for by a microprocessor or other type of digitally based circuit. As can be appreciated, the analog signal can be used without conversion to a digital signal. In a further embodiment, the system parasitic controller causes the switch controller to alter the electrical connection and disconnection sequence during the capacitance measuring sequence so as to detect the parasitic charge in the capacitance measuring circuit. In one aspect of this embodiment, the system parasitic controller causes the switch controller to disconnect the reference electrode from the ground and to cause the reference electrode to at least substantially follow the potential on the sensor electrode throughout the capacitance measuring sequence. Since the voltage or potential on the reference electrode is at least substantially identical to the voltage or potential on the sensor electrode, the potential on the reference electrode always follows the potential on the sensor electrode during the capacitance measuring sequence. As a result, any charge on the capacitor after the capacitance measuring sequence is completed represents the parasitic charge in the capacitance measuring circuit. In another aspect of this embodiment, the system parasitic controller causes the switch controller to ground the reference electrode throughout the capacitance measurement sequence. Since the reference electrode is always in a grounded state throughout the capacitance measuring sequence, the charge on the capacitor after the capacitance measurement sequence is completed represents the parasitic charge in the capacitance measuring circuit.
In still yet a further aspect of the present invention, a capacitance measuring circuit is positioned at each lateral edge of the material so as to measure the capacitance at each edge of the material. The measured capacitance from the two capacitance measuring circuits is used by a position controller to properly control the lateral position of the material. In one embodiment, the position controller is a comparison circuit or microprocessor which compares the two capacitances from the capacitance measuring circuit to adjust the lateral position of the material. In another embodiment, the electrodes of each capacitance measuring circuit are symmetrically positioned relative to the material so that the measured capacitance of each capacitance measuring circuit is generally the same when the material is in the proper lateral position. In one aspect of this embodiment, the position controller compares the measured capacitance of each capacitance measuring circuit and sends a signal to a lateral material positioner to cause the material to move laterally so as to cause the measured capacitance of each capacitance measuring circuit to be equal.
It is the primary object of the present invention to accurately measure the capacitance between a reference electrode and a sensor electrode when a material is at least partially positioned there between.
It is another object of the present invention to provide a capacitance measuring circuit which is simple and easy to operate.
It is yet another aspect of the present invention to provide a capacitance measuring circuit which charges and partially discharges a capacitor to obtain the capacitance between a reference electrode and a sensor electrode.
It is still another object of the present invention to provide a capacitance measuring circuit which includes an integrator circuit to integrate the charge on a capacitor over a series of capacitance measurement sequences.
It is a further object of the present invention to provide a capacitance measuring circuit which is used to detect the position of a material relative to a sensor electrode and a reference electrode.
It is still a further object of the present invention to provide a capacitance measuring circuit which is used in conjunction with another capacitance measuring circuit to detect the position of a material.
It is still yet a further object of the present invention to provide a capacitance measuring circuit which is used in conjunction with another capacitance measuring circuit to generate signals which control the position of the material.
It is another object of the present invention to provide a capacitance measuring circuit which detects the parasitic charge in one or more components of the capacitance measuring circuit.
It is yet another object of the present invention to provide a capacitance measuring circuit which compensates for the parasitic charge in the capacitance measuring circuit when measuring the capacitance between a reference electrode and a sensor electrode.
It is still yet another object of the present invention to provide a capacitance measuring circuit which shields a sensor electrode and/or a reference electrode from external electrical interference when measuring the capacitance between the electrodes.
These and other objects and advantages will become apparent to those skilled in the art upon reading and following this description taken together with the accompanied drawings.